Field of the Invention
Embodiments of the invention relate to an organic light emitting diode display.
Discussion of the Related Art
An organic light emitting diode (OLED) display is a self-emission display device. The OLED display may be manufactured to have lower power consumption and a thinner profile than a liquid crystal display requiring a backlight unit. Further, the OLED display has advantages of a wide viewing angle and a fast response time. As the development of process technology reaches large-sized screen mass production technology, the OLED display has expanded its market while competing with the liquid crystal display.
Each pixel of the OLED display includes an organic light emitting diode (OLED) having a self-emitting structure. As shown in FIG. 1, organic compound layers including a hole injection layer HIL, a hole transport layer HTL, an emission layer EML, an electron transport layer ETL, an electron injection layer EIL, etc. are stacked between an anode and a cathode of the OLED. The OLED display implements an input image using a phenomenon, in which the OLED emits light when electrons and holes are combined in an organic layer through a current flowing in a fluorescence or phosphorescence organic thin film.
The OLED display may be variously classified depending on kinds of emission materials, an emission method, an emission structure, a driving method, etc. The OLED display may be classified into a fluorescent emission type and a phosphorescent emission type depending on the emission method. Further, the OLED display may be classified into a top emission type and a bottom emission type based upon the emission structure. Also, the OLED display may be classified into a passive matrix OLED (PMOLED) display and an active matrix OLED (AMOLED) display depending on the driving method.
Each pixel of the OLED display includes a driving thin film transistor (TFT) controlling a driving current flowing in the OLED depending on data of the input image. Characteristics, such as a threshold voltage and a mobility, of the driving TFT have to be equally designed in all of the pixels of the OLED display, but are not uniform depending on a process deviation, a driving time, a driving environment, etc. Thus, the OLED display has adopted a compensation technology for sensing changes in driving characteristics of the pixels to properly change input data based on the sensing result. The changes in the driving characteristic of the pixel include changes in the characteristic of the driving TFT including the threshold voltage, the mobility, etc. of the driving TFT.
The changes in the driving characteristic of the pixel may be estimated based on changes in a source voltage of the driving TFT. However, because it takes much time to sense the characteristic of the driving TFT, it is difficult to secure a sensing time during a normal drive.
Time capable of sensing the characteristic of the driving TFT during the normal drive of the OLED display may be assigned within a vertical blank period, in which new data is not applied to the pixel. The vertical blank period is a period, in which there is no data enable signal DE between an Nth frame period and an (N+1)th frame period, where N is a positive integer. The data enable signal DE is synchronized with data of the input image to be displayed on a display panel. The data of the input image is not input in the vertical blank period. However, because a length of the vertical blank period is short, only changes in driving characteristics of sub-pixels of one color arranged in one line can be sensed during one vertical blank period. As a result, because an update cycle of a compensation value of the sub-pixels of each color in all of the pixels lengthens, the changes in the driving characteristic cannot be rapidly compensated.